Many metals and semi-metals form oxides, and some have a significant solubility for oxygen. In many cases, the oxygen is detrimental and therefore needs to be reduced or removed before the metal can be fully exploited for its mechanical or electrical properties. For example, titanium, zirconium and hafnium are highly reactive elements and, when exposed to oxygen-containing environments rapidly form an oxide layer, even at room temperature. This passivation is the basis of their outstanding corrosion resistance under oxidising conditions. However, this high reactivity has attendant disadvantages which have dominated the extraction and processing of these metals.
As well as oxidising at high temperatures in the conventional way to form an oxide scale, titanium and other elements have a significant solubility for oxygen and other metalloids (e.g. carbon and nitrogen) which results in a serious loss of ductility. This high reactivity of titanium and other Group IVA elements extends to reaction with refractory materials such as oxides, carbides etc. at elevated temperatures, again contaminating and embrittling the basis metal. This behaviour is extremely deleterious in the commercial extraction, melting and processing of the metals concerned.
Typically, extraction of a metal from the metal oxide is achieved by heating the oxide in the presence of a reducing agent (the reductant). The choice of reductant is determined by the comparative thermodynamics of the oxide and the reductant, specifically the free energy balance in the reducing reactions. This balance must be negative to provide the driving force for the reduction to proceed.
The reaction kinetics are influenced principally by the temperature of reduction and additionally by the chemical activities of the components involved. The latter is often an important feature in determining the efficiency of the process and the completeness of the reaction. For example, it is often found that although this reduction should in theory proceed to completion, the kinetics are considerably slowed down by the progressive lowering of the activities of the components involved. In the case of an oxide source material, this results in a residual content of oxygen (or another element that might be involved) which can be deleterious to the properties of the reduced metal, for example, in lower ductility, etc. This frequently leads to the need for further operations to refine the metal and remove the final residual impurities, to achieve high quality metal.
Because the reactivity of Group IVA elements is high, and the deleterious effect of residual impurities serious, extraction of these elements is not normally carried out from the oxide, but following preliminary chlorination, by reducing the chloride. Magnesium or sodium are often used as the reductant. In this way, the deleterious effects of residual oxygen are avoided. This inevitably leads, however, to higher costs which make the final metal more expensive, which limits its application and value to a potential user.
Despite the use of this process, contamination with oxygen still occurs. During processing at high temperatures, for example, a hard layer of oxygen-enriched material is formed beneath the more conventional oxide scale. In titanium alloys this is often called the “alpha case”, from the stabilising effect of oxygen on the alpha phase in alpha-beta alloys. If this layer is not removed, subsequent processing at room temperature can lead to the initiation of cracks in the hard and relatively brittle surface layer. These can then propagate into the body of the metal, beneath the alpha case. If the hard alpha case or cracked surface is not removed before further processing of the metal, or service of the product, there can be a serious reduction in performance, especially of the fatigue properties. Heat treatment in a reducing atmosphere is not available as a means of overcoming this problem because of the embrittlement of the Group IVA metals by hydrogen and because the oxide or “dissolved oxygen” cannot be reduced or minimised. The commercial costs of getting round this problem are significant.
In practice, for example, metal is often cleaned up after hot working by firstly removing the oxide scale by mechanical grinding, grit-blasting, or using a molten salt, followed by acid pickling, often in HNO3/HF mixtures to remove the oxygen-enriched layer of metal beneath the scale. These operations are costly in terms of loss of metal yield, consumables and not least in effluent treatment. To minimise scaling and the costs associated with the removal of the scale, hot working is carried out at as low a temperature as is practical. This, in itself, reduces plant productivity, as well as increasing the load on the plant due to the reduced workability of the material at lower temperatures. All of these factors increase the costs of processing.
In addition, acid pickling is not always easy to control, either in terms of hydrogen contamination of the metal, which leads to serious embrittlement problems, or in surface finish and dimensional control. The latter is especially important in the production of thin materials such as thin sheet, fine wire, etc.
It is evident therefore, that a process which can remove the oxide layer from a metal and additionally the dissolved oxygen of the sub-surface alpha case, without the grinding and pickling described above, could have considerable technical and economic benefits on metal processing, including metal extraction.
Such a process may also have advantages in ancillary steps of the purification treatment, or processing. For instance, the scrap turnings produced either during the mechanical removal of the alpha case, or machining to finished size, are difficult to recycle due to their high oxygen content and hardness, and the consequent effect on the chemical composition and increase in hardness of the metal into which they are recycled. Even greater advantages might accrue if material which had been in service at elevated temperatures and had been oxidised or contaminated with oxygen could be rejuvenated by a simple treatment. For example, the life of an aero-engine compressor blade or disc made from titanium alloy is constrained, to a certain extent, by the depth of the alpha case layer and the dangers of surface crack initiation and propagation into the body of the disc, leading to premature failure. In this instance, acid pickling and surface grinding are not possible options since a loss of dimension could not be tolerated. A technique which lowered the dissolved oxygen content without affecting the overall dimensions, especially in complex shapes, such as blades or compressor discs, would have obvious and very important economic benefits. Because of the greater effect of temperature on thermodynamic efficiency these benefits would be compounded if they allowed the discs to operate not just for longer times at the same temperature, but also possibly at higher temperatures where greater fuel efficiency of the aeroengine can be achieved.
In addition to titanium, a further metal of commercial interest is Germanium, which is a semi-conducting metalloid element found in Group IVA of the Periodic Table. It is used, in a highly purified state, in infra-red optics and electronics. Oxygen, phosphorus, arsenic, antimony and other metalloids are typical of the impurities which must be carefully controlled in Germanium to ensure an adequate performance. Silicon is a similar semiconductor and its electrical properties depend critically on its purity content. Controlled purity of the parent silicon or germanium is fundamentally important as a secure and reproducible basis, onto which the required electrical properties can be built up in computer chips, etc.
U.S. Pat. No. 5,211,775 discloses the use of calcium metal to deoxidise titanium. Okabe, Oishi and Ono (Met. Trans B. 23B (1992):583, have used a calcium-aluminium alloy to deoxidise titanium aluminide. Okabe, Nakamura, Oishi and Ono (Met. Trans B. 24B (1993):449) deoxidised titanium by electrochemically producing calcium from a calcium chloride melt, on the surface of titanium. Okabe, Devra, Oishi, Ono and Sadoway (Journal of Alloys and Compounds 237 (1996) 150) have deoxidised yttrium using a similar approach.
Ward et al, Journal of the Institute of Metals (1961) 90:612, describes an electrolytic treatment for the removal of various contaminating elements from molten copper during a refining process. The molten copper is treated in a cell with barium chloride as the electrolyte. The experiments show that sulphur can be removed using this process. However, the removal of oxygen is less certain, and the authors state that spontaneous non-electrolytic oxygen loss occurs, which may mask the extent of oxygen removal by this process. Furthermore, the process requires the metal to be molten, which adds to the overall cost of the refining process. The process is therefore unsuitable for a metal such as titanium which melts at 1660° C., and which has a highly reactive melt.